1. Field of the Invention
The present invention related to a charging device capable of providing backflow current and inrush current protection, and more particularly, to a charging device capable of providing backflow current and inrush current protection simultaneously by means of a dynamic protection unit.
2. Description of the Prior Art
With the tide of electronic devices turning towards smaller and lighter devices, portable electronic devices, such as mobile phones, cameras and personal digital assistants (PDAs), are used more and more frequently in daily life. For helping users to carry portable electronic devices at anytime and anywhere, the portable electronic devices are all equipped with rechargeable batteries for providing desired electric power when the portable electronic devices operate. In general, when the electric power of the rechargeable batteries are exhausted, the portable electronic devices can perform recharging procedures for the rechargeable batteries by connecting to an external power source through a connecting line, such as an universal serial bus (USB), so as to maintain effective operation of the portable electronic devices. When the portable electronic device is connected to an external power source, an internal power control circuit of the portable electronic device switches the power source from the rechargeable battery to the external power source. At such a time, the external power source not only supplies sufficient power to the portable electronic device for normal operation, but also enables the recharging procedures for the rechargeable battery to replenish consumed energy of the rechargeable battery until the portable electronic device disconnects from the external power source.
However, the above-mentioned recharging manner often results in the following two issues. First, when the portable electronic device is connected to an external power source for being recharged, at that instant an inrush current occurs that impacts internal circuits of the portable electronic device and the battery before the system is stable, with a result that the circuit components of the portable electronic device are damaged over a long period of time. Second, when the portable electronic device is removed from the external power source after completing recharging the battery, if the power control circuit cannot cut off the current paths immediately, a backflow current then occurs to flow back to the integrated circuits (ICs) of the portable electronic device from the rechargeable battery, resulting in error operations of the portable electronic device.
Thus, in order to solve the problems, U.S. Pat. No. 6,967,468 “Overvoltage and Backflow Current Protection for A Battery Charger” discloses a charging device, which increases a protection device between the current source and the rechargeable battery. When the charging voltage disappears, the protection device immediately cuts off the current path between the battery and the current source, such that the currents cannot flow back to other integrated circuits of the portable electronic device from the rechargeable battery. Please refer to FIG. 1. FIG. 1 is a schematic diagram of a charging device 10 disclosed in U.S. Pat. No. 6,967,468. As shown in FIG. 1, a transistor MP1 is a current source for providing charging currents, and a protection device 11 includes transistor MP2 and MN1 and resistors R1, R2 and R3. In a normal charging mode, the transistor MN1 is turned on for the resistors R1 and R2 capable of providing a bias voltage to the transistor MP2, and the transistor MP2 can further be turned on when the weighting of the resistors R1, R2 and R3 is appropriately adjusted, so that the charging device 10 can charge the rechargeable battery normally. When completing recharging the battery, the voltage of the external power source (+5V) disappears. At this time, the resistor R3 can provide a voltage from the battery for a gate electrode of the transistor MP2, and the transistor MN1 is turned off for blocking the bias voltage from the resistors R1 and R2, such that the transistor MP2 can be turned off immediately. Therefore, the current path between the transistor MP1 and the rechargeable battery is disconnected, so as to achieve the backflow current protection.
In the prior art, since the gate electrode of the transistor MP1 is still in a floating state at the instant when the charging device 10 is connected to the external power source (+5V), a conducting path will be formed in the transistors MP1 and MP2 before the system is stable, so that an uncontrollable inrush current may occur to damage the circuit elements of the portable electronic device. Besides, since the protection device 11 will generate a voltage difference between the transistor MP1 of the current source and the rechargeable battery, for preventing the transistor MP1 from being compressed into the triode region during the recharging process, the source-to-gate voltage Vsg of the transistor MP2 cannot be too great, so that the size of the transistor MP2 has to be increased. On the other hand, when recharging the battery, a leakage current will be generated through the resistors R1 and R2 to the ground. Thus, in order to prevent from too much leakage current, the values of the resistors have to be increased, so as to increase the layout area.
Therefore, the prior art provides the method for preventing from backflow current, but the prior art does not provide the inrush current protection at the same time. Besides, the prior art has to increase the layout area greatly, and thus the production cost is increased as well.